Hydraulic Return Filter: Functions, Types, and How to Choose

In hydraulic systems, contamination control determines the lifespan of the equipment, and the return line filter is a crucial component. This article will explain its practical knowledge, covering its function, structure, types, selection, installation, and maintenance.

What Is A Hydraulic Return Filter?

A hydraulic return filter is a device that filters contaminants from the hydraulic fluid in a hydraulic system. It is usually installed in the return line or inside the hydraulic tank.

What Does a Hydraulic Return Filter Do?

Hydraulic fluid will mix with dirt, dust, and wear debris as it flows through the various components of the hydraulic system.

The hydraulic return filter intercepts these contaminants and prevents them from re-entering the hydraulic system and causing damage to hydraulic pumps, valves, cylinders, and other components. In practice, it extends the service life of the equipment and reduces downtime.

How Does A Hydraulic Return Filter Work?

Hydraulic Oil Flows into the Return Pipe

After passing through the actuating elements, the hydraulic fluid will contain some dust, dirt, and wear particles. At this point, this oil mixed with contaminants will flow to the return line, ready to return to the tank for circulation.

Hydraulic Oil Enters the Hydraulic Return Filter

Hydraulic oil passes through a return filter before entering the tank; impurities in the fluid are adsorbed on the surface of the filter element, and the purified hydraulic oil flows into the tank.

Filter Blockage and Replacement

When the filter element of the hydraulic return filter attracts too many contaminants, it is easy to clog. At this point, the clogging indicator on the filter will sound an alarm, reminding you that it is time to change the filter element or clean the filter.

The Function of the Bypass Valve

To prevent the filter element from clogging and interfering with the operation of the system, most manufacturers of return line filters usually fit you with a bypass valve. When the filter element attracts too much contaminant, the pressure of the oil rises, and the bypass valve automatically opens, allowing some of the oil to flow directly into the tank around the filter element.

What Are the Main Components of A Hydraulic Return Filter?

Filter Housing

The housing is generally made of corrosion-resistant, heat-resistant metal. It wraps around the filter element and prevents damage to the filter element.

Inlet and Outlet Points

The inlet and outlet are where the filter is connected to the oil return line and the tank. The oil will enter the filter from the inlet, be filtered, and then flow from the outlet to the tank.

Filter Element

The filter element is the part of the filter that is actually responsible for filtration. It consists of a fiber material, a metal mesh, and a paper cartridge. It absorbs dust, dirt, and wear particles from the oil.

Bypass Valve

The bypass valve is a safety component of the hydraulic return filter. It opens automatically when the filter becomes clogged, preventing excessive pressure in the line.

Clogging Indicator

The clogging indicator is a device attached to the filter element. When the filter element becomes clogged with contaminants to a certain degree, it will sound an alarm to remind you to clean or replace the filter element.

What Types of Hydraulic Return Filters Are Available?

In-Line Return Filters

Straight-through oil return filters are installed in the oil return pipeline of the hydraulic system. The oil is filtered directly through the filter.

It has a simple structure, a small size, a simple installation method, very convenient for daily maintenance.

You can install it in hydraulic equipment where space is at a premium.

Tank-Mounted Return Filters

The tank-mounted return filter is mounted on the top or side wall of the hydraulic tank. The fluid is filtered through this filter before flowing into the tank.

These filters are generally larger in size and are able to filter more hydraulic fluid with better stability.

You will often see it among construction machinery, mining equipment, and heavy industrial equipment.

Spin-On Return Filters

Spin-on oil return filters have a one-piece cartridge and housing. You can replace the entire filter by simply rotating it.

It is simple, low-cost, and easy to remove and replace.

It is ideal for installation in small and medium-sized equipment, such as automobiles and agricultural machinery.

Element-Type Filters

The split oil return filter has a separate housing and filter element. You can replace the filter element separately. The housing can be cleaned and then continued to be used.

It is designed to be more flexible. You can replace the cartridge with different sizes and precision according to your actual needs.

If you need to use it on precision manufacturing equipment, servo control systems, and other machines that require high precision, then the split oil return filter is a good choice.

Duplex Filters

The double cartridge filter is equipped with two independent filter units, which can be switched automatically. When one of the filter elements becomes clogged, the hydraulic fluid flows to the other filter unit for filtration.

This filter allows the system to continue to run while you change or clean the filter.

If you have machines that need to run continuously for a long time with high downtime costs, such as hydraulics in industrial production lines, equipment in the energy and power industry, as well as offshore and petroleum equipment, you have to use a double cartridge filter.

Where Are Hydraulic Return Filters Commonly Used?

The following table lists the main applications of the hydraulic return filter in different industries.

How to Choose The Right Hydraulic Return Filter?

Step 1: Get your target ISO cleanliness level

First, you need to find the component of the system that requires the highest level of cleanliness (Servo Valve > Proportional Valve > Piston Pump/Motor > Gear Pump).

Then you need to go to the manufacturer’s chart and find the required cleanliness level for this part. (For example: Required cleanliness. ISO 18/16/13, Oil cleanliness shall be ISO 16/14/11 or better.)

If the information is not available, you can determine it by system type. The following table can help you.

Step 2: Back Extrapolate the Accuracy Range by ISO Grade

After determining your target ISO rating, you can use the table below to determine the accuracy range of the filter.

Step 3: Check the β-value of the Cartridge

After determining the accuracy range of the oil return filter, you also need to find the β-value in the technical parameters of the product (e.g., 10 μm β₁₀ ≥ 200, 10 μm β₁₀ ≥ 75). The β-value refers to the intercepting efficiency of the cartridge, and the higher the β-value is, the higher the intercepting efficiency.

The following table shows what the different beta values represent.

Example: 10μm β₁₀ ≥ 200 means that the filter element has an interception efficiency of ≥ 99.5% for particles with a diameter of ≥ 10μm.

Step 4: Verify Flow Rate and Pressure Drop

After determining the filtration accuracy and beta value, you also need to check the flow rate and pressure drop of the system to make sure that the filter does not cause too much obstruction to the flow of hydraulic fluid in the system.

First, you need to determine the maximum return flow rate of the system. Then find the Flow vs ΔP curve for the filter and look at the corresponding pressure drop at the point of maximum return flow to determine if the pressure drop is reasonable.

The judgment criteria are listed in the table below:

For example, if you find that the pressure drop is greater than 0.3 bar when you change a new filter, it means that the filter has caused a large obstruction to the flow of hydraulic oil. At this point, you need to use a larger filter or choose a slightly less accurate filter.

Step 5: Make Minor Adjustments According to the Intensity of System Contamination

The cleanliness of the environment is also an important factor in the process. You need to adjust the selection and configuration of the hydraulic return filter according to the degree of contamination in the system.

The table below gives you a list of suggestions.

Step 6：Always Consider the Bypass Valve and the Clogging Indicator

In fact, all return oil filters will gradually become clogged over time. That’s why it’s a good idea for you to choose a hydraulic return filter that is equipped with a bypass valve and a clogging indicator.

Step 7：Select the Filter Type Based on Installation and Maintenance Conditions

In addition to the above factors, when choosing a hydraulic return filter, consider your installation conditions and the ability to maintain.

The table below will help you quickly determine which type you should choose.

How to Install A Hydraulic Return Filter?

General Installation Process

Step 1: Determine the Installation Location

In my experience, installing the Hydraulic Return Filter in a location where the return oil is the last hurdle before it enters the tank works best. This prevents contaminants from entering the tank and allows the system to run more consistently.

Step 2: Check and Prepare the Mounting Surface and Space

Before installation, you have to make sure that the contact surface is flat and the sealing groove is intact. You’ll also want to leave enough room for changing the cartridge at a later date.

Step 3: Fix the Filter Body

Different types of Hydraulic Return Filter are not installed in quite the same way. Regardless of the type, however, the filter must be secured individually and not suspended directly from the piping. This will cause the piping to wobble, and the filtration will be less effective.

In addition, you should apply even pressure when securing to avoid deforming the housing. Doing so will prevent the filter from leaking.

Step 4: Connect the Oil Return Line and Confirm the Flow Direction

When connecting the return line, you should install it in the direction of flow (IN / OUT or arrow) marked on the filter housing; don’t get it backwards.

When connecting the return line to the filter, tighten the fittings but do not lock them. If you find that the joints don’t fit, don’t force them together; this will greatly reduce the effectiveness of the filter.

Step 5: Make Sure the Oil Return Drop Point is below the Oil Level

When installing the filter, you should allow the filtered fluid to enter the tank below the oil level. You can use a downpipe, deflector, or diffuser to do this.

If the oil enters the tank above the oil level, it can easily cause the oil to foam and emulsify. Doing so may also allow air to be drawn into the hydraulic pump, creating a lot of noise.

Step 6: Install Cartridges and Seals

There are a few things you need to keep in mind when installing cartridges and seals.

You should make sure that the cartridge is the correct type.

You should clean the inside of the housing before inserting the cartridge.

Before installing the seals, you need to apply a small amount of hydraulic oil to the surface

You should make sure that the seals are not twisted and pinched

Step 7: Install Clogging Indicator and Bypass Assembly

When installing the blockage indicator and bypass assembly, you must follow the instructions on the plans. If you are using an appliance-type indicator, the wiring needs to be properly mated.

Step 8: Pre- and Post-startup Checks

Before starting, you should make sure that all the installation steps are all completed. The cartridge has been installed. Seals are installed in each port.

After startup, you should run the system at a lower speed first to expel any internal air. Then check the filter for oil leaks, abnormal vibrations, and noise. You should also observe the status of the clogging indicator to ensure that the filter is not clogged.

Different Types of Hydraulic Return Filter Installation Methods and Precautions

Tank-Mounted Return Filter

This type of oil return filter is usually fixed directly to the tank wall or top of the tank by a flange.

You should tighten the bolts of the flange evenly to avoid leakage of the sealing surface due to uneven stress.

In-Line Return Filter

This oil return filter is installed in the middle of the oil return line by means of a series connection.

You must use a separate bracket to hold it in place, and you must not let the piping bear the full weight of it.

You must not install it in a place where there is too much vibration.

Be careful not to bend the piping too much when arranging it.

Spin-On Return Filter

This oil return filter is screwed directly onto the base by means of threads.

You can only screw it on by hand and cannot use tools to tighten it violently.

You must periodically check the screw-in area for looseness or leakage.

Element-Type Return Filter

This oil return filter needs to be installed with the housing fixed first. Later, you only need to replace the internal filter element during maintenance.

You should make sure that the position of the filter element is aligned with the guide structure and that there is no offset.

You should clean the sealing surface of the end cap to prevent debris from mixing with the filter.

You should tighten the end cap bolts evenly in a diagonal sequence.

When replacing the filter element, you should check and replace the seals at the same time.

Duplex Return Filter

This oil return filter requires two filter housings to be installed in parallel.

You must reserve space for the switching handle or valve body to operate.

You must make sure that the specifications of the filter elements on both sides are the same.

After installation, you should test the switching action for smoothness.

How to Maintain a Hydraulic Return Filter Properly?

When Should I Maintain?

If your clogging indicator alarms, it’s time to replace the filter element.

When the cartridge is severely clogged, the bypass valve opens to allow the oil to bypass the filter element and return to the tank. This time also means that you need to replace the filter element.

You should also go ahead with maintenance if you notice that the fluid is getting dirty and the oil has unusual metal particles in it.

Standard Maintenance Procedures

Step 1: Shutdown and Pressure Relief

You should shut down the hydraulic system to release any residual pressure in the system.

Step 2: Disassemble the Filter and Take out the Old Cartridge

You will need to open the end cap or unscrew the cartridge and remove the old cartridge.

You will need to catch the cartridge in a container to prevent hydraulic fluid from dripping onto other equipment.

Step 3: Check the Old Cartridge

Don’t be in a hurry to throw away the old cartridge after you take it off; take a look at it first.

If it contains a lot of metal shavings or unusual particles, it means the internal wear and tear is serious and needs to be checked and repaired promptly.

Step 4: Clean the Housing and Check The Seals

Next, you need to remove the filter housing for cleaning and inspect the seals for deterioration, distortion, or breakage.

Step 5: Installation of New Cartridge and Seals

When installing a cartridge, you should make sure it is the correct type.

When installing a seal, you should apply a coat of clean hydraulic oil to the seal.

Step 6: Start-up and Acceptance of the System

Once the installation is complete, you should start the system and remove any air from the system.

You should then check the system for oil seepage and watch for the clogging indicator to return to normal.

Maintenance is complete when the system is not leaking oil and the indicator is not alarming.

Three Better Suggestions

Don’t just change the Cartridge by the Time.

Many manuals will tell you above that you need to change the filter element once every three months, but you don’t want to do that.

You need to consider the timing of filter element replacement based on a combination of what the clogging indicator says, the condition of the fluid, and the intensity of use.

Record Data for Each Maintenance

At each maintenance visit, you’d be wise to keep track of the time of replacement, the cartridge model, the alarm status, and the number of hours it’s been running. By doing this, you will know if the cartridge life is getting shorter.

The Solution to the Cartridge Clogging Too Fast

If you find that the cartridge is clogged not long after it has been changed, don’t rush to change the cartridge, but first check to see if the breather is failing, if the fueling method is clean, and if the seals are deteriorating. Wait for all these issues to be resolved before replacing the cartridge.

FAQ

What is the essential difference between an oil return filter and a pressure filter?

The oil return filter is installed on the oil return line. It prevents contaminants from returning to the tank and then circulating into the system.

The pressure filter is installed on the pump outlet or main oil line. It prevents precision components from being damaged by contaminants.

They are located in different places, and the focus of protection is different.

What is the relationship between an oil return filter and a tank breather?

The tank breather is responsible for filtering the air entering the tank. The oil return filter is responsible for filtering contaminants from the oil return line.

If the breather fails, dirt will enter the tank, and the return filter will clog faster.

They determine how clean the hydraulic system is.

Why does the oil return filter alarm so often, but the oil doesn’t look dirty?

• If this happens, it can be caused by the following:
• The precision of the filter element is too fine
• The oil return flow is too high
• The viscosity of the oil is too high
• The filter area of the filter element is too small

How to determine whether the cartridge is “normally clogged” or “abnormally clogged”?

If the cartridge replacement cycle has been relatively stable, and the clogging usually occurs only at the end of the cycle, then it is generally normal.

If the cartridge starts to alarm shortly after it is replaced, it means that the cartridge selection is not quite right or there is a problem with the contamination control of the system.

Can a cheap oil return filter really affect system life?

Possibly. Cheap oil return filters often have the following problems:

• Inconistent filtration efficiency
• Poor sealing
• Filtration capacity does not match what is written on the standard

This can cause contamination of the system to become more severe and shorten the life of pumps, valves, and other components.

Do oil return filters need to be specially configured for high-dust environments?

Yes. In high-dust environments, you need to go with a larger oil return filter, with a more efficient tank breather, and shorten the cartridge replacement intervals. Otherwise, the cartridge is prone to clogging very quickly.

Is there a general industry standard for oil return filter selection?

There are reference standards, but you can’t copy them.

Common references include:

• ISO 4406 Fluid Cleanliness Classes
• Cleanliness Requirements for Critical Components
• Hydraulic Filter Specifications

You should ultimately consider this in the context of your system architecture, operating environment, and maintenance capabilities.

Final Thoughts

The hydraulic return filter is small, but it can determine the cleanliness and service life of the hydraulic system. From selection, installation, to maintenance, every detail will affect the long-term stable operation of the system. You can only truly understand and reasonably use the oil return filter to reduce the risk of failure and downtime from the source.

Target Hydraulics has more than 10 years of experience in hydraulic return filter application and selection, and can provide you with professional product selection, system matching, and technical support services. Welcome to contact us to customize a more reliable filtration solution for your hydraulic system!